This Small Business Innovation Research Phase I project will develop and demonstrate a commercially-feasible Artificial Accessory Cell Bioreactor (AACB) that can be used for translational research applications and scale-up of stem cell cultures to support the development of emerging cell therapy applications. The proposed bioreactor has two unique aspects. The first component is a polymer microsphere-based artificial accessory cell that will serve as a partial/complete replacement for soluble cytokines and to mimic the stem cell niche for the system of interest. The second component is a unique and extremely easy-to-use media-replenishing bioreactor. The Artificial Accessory Cells (AACs) will consist of polymer microspheres (6 um), functionalized with one or multiple cytokine, peptide mimetic, ECM, and/or other molecule depending on the needs of the particular stem cell culture system. The focus of this proposal is the development and testing of the unique bioreactor which will incorporate use of the AACs as a key feature in advancing stem cell translational research. While the AACB technology will be applicable to all types of stem cells for which some information on its biological niche is available, the initial technology demonstration and suite of products will be focused on hematopoietic stem cells (HSCs) because of the extensive knowledge and range of cytokines available for this well-studied system. Immobilization of biologically active factors is essential to achieving the goal of efficiently directing stem cells along desired pathways for therapeutic aims. In vivo, many important cytokines are presented in membrane-bound or extra-cellular matrix (ECM)-bound forms (1). Presentation in this format often induces a unique biological response on the target cell that cannot be duplicated using soluble forms of the molecule. In addition, this strategy may be effective for limiting the quantit of expensive signaling molecules necessary for large- scale production of differentiated cells, such as the production of red blood cells for transfusion in an ex vivo format. The key to the proposed AACB product's success will be process consistency, ease-of-use, and the development of approaches, protocols and techniques that allow for the use of multiple cytokine types, feed rates, and modes of operation across a wide range of stem cell types.